How to attach a Mesh to armature while keeping transform
Thats actually pretty simple, your mesh is parented to the armature in its edit mode state, pose mode is meant to pose your model after it has been rigged correctly.

even if you see the skeleton in a T pose you are actually parenting your T pose (with arms stretched out, like a T) to a rig with its arms in 45°.

you can check this by resetting to rest pose, and seeing how your mesh will be intact but the skeleton will still be offset by the same amount.

As this happens then we apply your rotation of the arms 45° in pose mode, thus rotating the arms 45° more.

What you need to do is either adapt the rig or the mesh before parenting. I suggest you reset your transforms in pose mode to properly see the state of your rig, to reset transforms press ALT+G for location, ALT+R rotation and ALT+S scale.